Planar lightwave circuits, including optoelectronic integrated circuits (OEICs), are currently used in optical telecommunication systems, inter alia, for allowing the branching, coupling, switching, wavelength multiplexing and wavelength demultiplexing of optical signals without intermediate transformation between optical and electrical media.
A planar lightwave circuit is typically fabricated by forming one or more waveguides on a planar substrate, such as a silicon wafer. More specifically, the waveguides are usually embedded within an optical layer that may consist of buffer layers, cladding layers, core layers and encapsulation layers formed on the planar substrate which is frequently formed from doped/undoped silica, LiNbO3, InP, GaAs, and/or polymer (including thermo-optic and electro-optic polymers).
Typically, a large number of PLCs are fabricated on a single wafer, which is subsequently cut into multiple PLC dies. These chips are then packaged according to their particular applications to form the corresponding PLC device. For example, depending upon the required application, the PLC die may include integrated optical components, such as modulators, optical switches, and wavelength multiplexer/demultiplexers, that are mounted on a sub-mount. Typically, the sub-mount is secured in an enclosure that facilitates the attachment of optical fibers or waveguides to the packaged PLC device.
Notably, the optical performance of PLCs are typically very sensitive to the internal temperature within the package enclosure and the stress of the packaged product. The temperature sensitivity relates, in part, to the temperature dependent refractive indices of the optical layers used to fabricate the various components, and in part, due to the significant coefficients of expansion of the materials typically used to form the PLC device. The stress sensitivity relates to the stress-dependent refractive index variation of the optical layers. Ambient temperature changes often result in the expansion or contraction of the optical layer, the substrate, and/or the sub-mount. In general, this expansion will result in the length and/or width of the waveguides being altered, thus affecting performance and reliability of the device. The stress induced in the optical layer from these expansive/contractive forces induce changes in the refractive index, and as a result modify the PLC performance characteristics.
An arrayed waveguide grating (AWG) is one example of a PLC that exhibits high sensitivity to both temperature and stress. An AWG functions as either a multiplexer or a demultiplexer. When functioning as a demultiplexer, the AWG takes a multiplexed optical signal transmitted from an input waveguide and transmits it through an array of curved waveguides such that a plurality of demultiplexed optical signals are individually output from a series of output waveguides. The optical path difference (OPD) between adjacent waveguides in the array provides the necessary interference to demultiplex the optical signal. Accordingly, the refractive index and lengths and widths of the arrayed waveguides must be accurately maintained. Even small changes in temperature will result in the central wavelength of each demultiplexed optical signal being shifted. For example, in silica-based AWGs the shift in central wavelength, due to temperature alone, is in the order of +0.011 nm/° C., in InP-based AWGs the shift in central wavelength is in the order of +0.019 nm/° C., and in polymer-based AWGs the shift in central wavelength is in the order of −0.1 nm/° C.
Conventionally, the thermal stability problems of PLCs have been countered by including a precise active thermal regulator with the PLC packaging. The thermal regulator typically includes a thermometer, a Peltier device, and a control unit for heating and/or cooling the PLC to maintain a constant temperature thereof. Since the PLC is maintained at a constant temperature, the problems associated with temperature sensitivity are greatly reduced. Unfortunately, to achieve an acceptable performance level, the thermal regulator is typically relatively costly, energy consuming, and requires additional circuitry/hardware to monitor and maintain a stable temperature.
The use of passive techniques for thermal compensation to reduce temperature sensitivity is advantageous over active compensation because: a) passive techniques are more reliable and less expensive; b) passive techniques do not require power; c) the package does not require heat sinks and may be made smaller; and d) the packaging process is faster and easier, since the extra components do not need to be placed or bonded.
Alternatively, PLCs have been designed to reduce temperature sensitivity and obviate the need for a thermal regulator. For example, U.S. Pat. No. 6,519,380, Dawes et al. discloses an organic containing overclad (such as a polymer or sol-gel material) in the silica optical layer. The organic containing overclad, which only needs to cover part of the PLC, is selected to have a negative variation in refractive index with temperature to compensate the positive variation in refractive index of the rest of the optical layer (i.e., the light also travels through the overclad). The result is an inhibited shift in central wavelength with temperature. U.S. Pat. No. 6,574,409, issued to Moroni et al., teaches an optical layer with a compensating region therein. More specifically, a region of the silica optical layer, which has a positive variation of index of refraction with temperature, is etched out and replaced with a polymer having a negative variation of index of refraction. The two different varying indices of refraction compensate for one another to reduce temperature sensitivity.
The two above-mentioned references propose performance stability by compensating for the variation of refractive index with temperature. Alternatively, it is possible to impart temperature insensitivity by compensating for the variation in waveguide dimensions with temperature. For example, U.S. Pat. No. 6,477,308, issued to Hattori et al., discloses an optical layer that has a positive linear expansion coefficient and a second layer adjacent to the optical layer that has a negative linear expansion coefficient. As the optical layer expands with temperature, the second layer begins to contract, thus compensating for any variation in length of the waveguides. U.S. Pat. No. 6,542,685, issued to Yoneda, discloses a substrate with a zero or negative coefficient of thermal expansion to obtain a temperature insensitive optical waveguide. In addition, the Yoneda reference teaches providing a second substrate having a zero or negative coefficient of thermal expansion that is mounted to the optical layer, while the original substrate is removed.
Unfortunately, each of aforementioned solutions require a modification to the PLC prior to packaging, thereby increasing the manufacturing time and cost. Furthermore, the modifications requiring the substrate to have a low or negative coefficient of expansion will complicate the design and manufacture of the corresponding PLC, and will further increase the manufacturing cost.
U.S. Pat. No. 5,978,539, issued to Davies et al., which is incorporated herein by reference, teaches the elimination of thermal effects on the optical properties of an unmodified PLC. More specifically, they teach mounting a control layer to the bottom of the silicon substrate, wherein the control layer and substrate both have a positive, but different coefficient of thermal expansion. In response to an increase in ambient temperature, the control layer expands at a faster rate than the substrate to create a non-planar substrate distortion, thus creating a temperature independent optical path length. Unfortunately, non-planar thermal distortions are associated with being undesirable for various reasons. For example, in U.S. Pat. No. 6,603,916, hereby incorporated by reference, McGreer et al. focus on methods to reduce similar non-planar distortions.
Notably, each of the above-mentioned references are based on reducing temperature sensitivity of silica-based PLCs. It is also possible to use the temperature sensitivity of PLCs to tune the device operation of the same. Any optical substrate has a variation of the refractive index with temperature through the coefficient dn/dT: ΔnT=dn/dT*ΔT. If the variation in the refractive index with temperature is not sufficient or is too large for a specific application, the effect can be corrected by adding to the variation of the refractive index through stress, e.g. thermal or mechanical stress, ΔnS, in a controlled way through the package base design in such a way that the total variation ΔnTotal=ΔnT+ΔnS has the desired value. Through fine tuning the ΔnTotal can be decreased.
More recently, there as been an increased interest in polymer-based PLCs, which are associated with lower production costs. However, polymer-based PLCs typically exhibit a negative and much larger variation of refractive index with temperature than do silica-based PLCs. Accordingly, accounting for temperature sensitivity, such as when maintaining device performance and/or tuning device operation, is often very challenging. In particular, most of the techniques used to impart temperature insensitivity to silica-based PLCs are not compatible with polymer-based PLCs, because they are used to compensate for positive variations in refractive index as opposed to the negative variation of refractive index with temperature exhibited by most polymer-based PLCs.
An object of the present invention is to overcome the shortcomings of the prior art by providing a package for inducing stress in a PLC, either passively to compensate for temperature variations or actively using thermally or mechanically actuated systems.